Naval and commercial vessels generate large volumes of oily wastewater, mostly in the form of bilge water and ballast water. Bilge water typically contains various oils and fuels, grease, antifreeze, hydraulic fluids, cleaning and degreasing solvents, detergents, rags, and metals (including arsenic, copper, cadmium, chromium, lead, nickel, silver, mercury, selenium, and zinc) that collect during the daily operation of a vessel. Bilge water may also contain “gray water,” which includes galley water; turbid water from showers and laundry; and drainage water from air conditioning units, drinking fountains, and deck drains. Ballast water may be contaminated with oil that was transported in the ship prior to ballasting, or may contain small animal and vegetable sea life drawn in with the ballast water.
Other smaller sources of oily wastewater generated onboard ships include steam condensate, boiler blowdown, elevator pit effluent, deck runoff, gas turbine wash water, motor gasoline compensating discharge, and aqueous wastes from other diverse types of machinery and machine operations.
In the past, these oily wastewaters were either stored for subsequent onshore treatment or simply discharged overboard. More recently, regulating bodies such as MARPOL, the EPA, the U.S. Coast Guard, the Department of Defense, and some states have enacted more stringent restrictions on the location and extent of such discharges. These new regulations require oily wastewater to be treated to 15 ppm or less oil content prior to overboard discharge. Some regions have yet more stringent requirements. For example, Canadian regulations in the Great Lakes limit oil content of discharged waters to 5 ppm. Uniform National Discharge Standards (UNDS) for vessels of the armed forces, now being developed in the United States under a three-phase program, may require numerous possible discharge streams to be controlled, and may be expanded to include additional pollutants, such as metals, as well as to civilian shipping.
The current state of the art is to hold wastewater in a storage tank for the duration of the voyage (and to treat it later onshore), or to use oil/water separators (OWS), usually of the parallel-plate type, to treat water on the ship. OWS systems are gravity separators that separate based on the different densities of oil and water phases. Under appropriate conditions, such separators can provide reasonably good separation of discrete oil and water phases. They are ineffective, however, in removing colloidal particles, emulsified oil or dissolved oil. Since oil in these forms is usually present at least at the hundreds of ppm level, oil/water separators are unable in meet the 15 ppm limit in most cases.
Both storage and simple gravity separation obviously have many drawbacks, and a clear need for better treatment techniques exists.
The U.S. Navy has installed separation systems using ceramic ultrafiltration membranes on a few vessels. When clean, the membrane systems have sufficient separation capability to meet the 15 ppm oil in wastewater discharge requirement. However, they are very susceptible to internal fouling (plugging of pores by oil or other contaminants) and surface fouling (build-up of an oil layer on the surface of the membrane). As a result, the membranes must be taken off-line and back-flushed or otherwise cleaned every day. Cleaning gradually becomes less effective, and the transmembrane water flux may decline to a level at which more water is being generated than can be treated.
Thus, better solutions to the water treatment problems of ship operators are urgently needed.
Combinations of unit separation steps, such as various forms of phase separation and membranes, as a general form of treatment for aqueous effluents of all kinds, are known in the prior art. For example, U.S. Pat. No. 4,915,844, to Nitto Denko, describes a combination of ultrafiltration membrane separation followed by centrifugal separation, or alternatively, ultrafiltration and centrifugation steps independent of each other, for recovering silica particles from process wastewater. U.S. Pat. No. 5,482,634, to Dow Chemical, describes the separation of cellulose ethers from water with a combination of centrifugation and ultrafiltration using glassy polymer membranes. U.S. Pat. Nos. 5,087,370 and 5,221,480, both to Clean Harbors, describe removal of toxic metals and organics from water by a combination centrifuge/membrane process, using a porous microfiltration or ultrafiltration membrane.
Phase separation combined with ultrafiltration has been described for treatment of diverse oily wastewater streams. U.S. Pat. No. 5,527,974, to Henkel Kommanditgesellschaft, describes separating natural fats and oils from glycerol water by a combination phase separation and microfiltration process. This patent also includes discussion of the need for periodic back-flushing of the membrane to reduce fouling. U.S. Pat. No. 5,501,741, to USS-POSCO, describes separating fats or fatty acids from water using either ultrafiltration membrane separation or centrifugation, followed by a microfiltration membrane step. British Patent GB1456304, to Abcor, describes separation of oil-water mixtures by a combination of ultrafiltration/centrifugation using a porous cellulose acetate membrane. U.S. Pat. No. 6,187,197, to Haddock, describes the use of a combination hydrocyclone/nanofiltration process. The process is described as a pretreatment for the standard reverse osmosis treatment used to separate oils, fuels, and dissolved solids from ethylene glycol/water engine coolant.
Other patents that disclose the combination of gravity separation and membrane filtration include U.S. Pat. No. 4,978,454, to Exxon, which describes a system using a gravity settler to recover a light phase and a heavy phase from a three-phase mixture, and a membrane to separate the intermediate phase. U.S. Pat. No. 5,108,549, to GKSS, describes a decanter/pervaporation process for separating organics from water.
Finally, U.S. Pat. No. 5,932,091, to the U.S. Secretary of the Navy, describes separating oily bilge water with a ceramic ultrafiltration membrane, which is backflushed after each wastewater treatment cycle to reverse the effects of fouling.
All the ultrafiltration membranes cited above are porous and are subject to severe internal and surface fouling by oil and particulate matter in the wastewater stream. Internal fouling of the pores of the membrane is usually irreversible. This type of fouling can be postponed by extensive pretreatment of the feed stream and repeated cleaning of the membrane. Over time, however, the pores of the membrane become permanently plugged, and the membrane must be replaced.
Surface fouling by deposition of solid material on the surface of the membrane can be reduced by high turbulence, regular cleaning, and using hydrophilic membrane materials to minimize adhesion to the membrane surface. Thus, any process using typical porous ultrafiltration membranes must endure periodic shutdowns while the membrane elements are taken off-line for treatment with appropriate cleaning solutions. However, such shutdowns are inconvenient, disruptive, and costly, and the cleaning procedures may be difficult to apply and only partially effective. Further, the spent cleaning solutions create yet another waste stream requiring treatment. Thus, such cleaning techniques are inappropriate for shipboard use. In addition, the composition of shipboard bilge and ballast waters can vary widely during a day of ship's operation, and the membranes may be suddenly subjected to a broad range of highly-fouling oil-water emulsions, solvents, surfactants and particulates, causing erratic or unpredictable membrane performance.
Attempts to use dense, nonporous membranes as reverse osmosis or ultrafiltration membranes have been reported in the literature. U.S. Pat. No. 5,265,734, to Kiryat Weitzman, describes a process for separating organic mixtures using an ethylenically unsaturated nitrile membrane coated with silicone to create a nonporous layer. This membrane is reported to be solvent resistant and to swell only minimally in the presence of organic solvents. U.S. Pat. No. 4,748,288, to Shell Oil, describes the use of a dense halogen-substituted silicone membrane to separate dissolved hydrocarbons from solvents. This membrane, also, is reported to be solvent stable and minimally swelling.
Such dense, nonporous membranes have been reported to be fouling resistant. An article by K. Ebert et al., “Solvent resistant nanofiltration membranes in edible oil processing,” (Membrane Technology, No. 107, p. 5–8), compares the performance of polyether-polyamide block copolymer membranes and cellulose-type membranes for separation of edible oils from solvents. An article by S. Nunes et al., “Dense hydrophilic composite membranes for ultrafiltration,” (J. Membrane Science, Vol. 106, p. 49–56, 1995), compares the separation performance and fouling resistance of polyether-polyamide block copolymer membranes and cellulose membranes for separating oil-water emulsions from the metal working industry. German Patent DE4237604, to GKSS, discloses the uses of polyether-polyamide block copolymer membranes or epichlorohydrin-ethylene oxide copolymer membranes for ultrafiltration applications, and notes their low tendency to fouling.
It is an object of the present invention to provide a membrane-based process for separation of oils from bilge, ballast, and other oily wastewaters generated in connection with naval and commercial shipping activities.
Additional objects and advantages of the invention will be apparent from the description below to those of ordinary skill in the art.